Salmonellae are Gram-negative, facultative anaerobic, motile, non-lactose is fermenting rods belonging to the family Enterobacteriaceae. Salmonellae are usually transmitted to humans by the consumption of contaminated foods and cause salmonellosis.
Salmonellae have been isolated from many animal species including, birds, cattle, sheep, pigs, dogs, cats, horses, donkeys, seals and reptiles. Ninety-five percent or more of the Salmonella serovars (ser.) isolated from food producing animals belong to Salmonella enterica subspecies enterica (S. enterica), with Salmonella ser. Typhimurium (S. Typhimurium), Salmonella ser. Choleraesuis (S. Choleraesuis), Salmonella ser. Derby (S. Derby), Salmonella ser. Infantis (S. Infantis), Salmonella ser. Bredeney (S. Bredeney), Salmonella ser. Rissen (S. Rissen), and Salmonella ser. Anatum (S. Anatum), as the most common serovars in pigs. Salmonella Enteritidis (S. Enteritidis), S. Typhimurium, Salmonella Hadar (S. Hadar), Salmonella Virchow (S. Virchow), S. infantis, Salmonella Kentucky (S. Kentucky), S. Bredeney, Salmonella Agona (S. Agona) and Salmonella paratyphi B (S. paratyphi B) are the most common in poultry.
Salmonella infections are a serious medical and veterinary problem world-wide and cause concern in the food industry. Control of salmonellosis is important to avoid potentially lethal human infections and considerable economic losses for the social security and animal husbandry industry.
There has been a long history of the use of live attenuated Salmonella vaccines as effective vaccines for the prevention of salmonellosis in animals and humans. The live attenuated oral typhoid vaccine, Ty21a (Vivotif®), manufactured by the Swiss Serum Vaccine Institute, has proved to be a successful vaccine for the prevention of typhoid fever and has been licensed in many countries including the US and Europe. However, none of the currently available vaccines confer any protection against recrudescence of infection e.g. triggered by stress (Nakamura et al., 1994; Wallis, 2001; Boyen et al., 2008).
Fasting and transportation of animals is known to cause varying levels of stress, depending on a number of parameters, such as crowding, temperature, social status, and duration of feed deprival/transport. A period of stress results in the release of a variety of neurotransmitters, peptides, cytokines, hormones, and other factors into the circulation or tissues of the stressed organism (Freestone et al., 2008). Besides the fast-acting catecholamines, which are released by the sympathetic nervous system, the hypothalamic-pituitary-adrenal axis becomes activated which results in the release of the slow-acting glucocorticoids by the adrenal gland (Dhabhar F S, 2009). These stress hormones can affect the host immune response via the modulation of various aspects of the immune system. However, the pathogenesis of an infection can also be altered by direct effects of these stress mediators on the bacteria. Bacteria can exploit the neuroendocrine alteration of a host stress reaction as a signal for growth and pathogenic processes (Freestone et al., 2008; Lyte M, 2004). This could partly explain the stress induced recrudescence of Salmonella Typhimurium by pigs (Boyen et al., 2009b).
Pigs secrete cortisol as the predominant glucocorticoid (Worsaae and Schmidt, 1980). Peak levels of cortisol occur immediately after start of transport and remain elevated throughout transport (Bradshaw et al., 1996), and plasma cortisol concentrations are an important measure of stress (Jensen-Waern and Nyberg, 1993).
Stress can increase Salmonella shedding in infected pigs and even cause a recrudescence of Salmonella in carriers (Hurd et al., 2002). Consequently, periods of stress result in increased cross-contamination during transport and lairage and to a higher degree of carcass contamination which could lead to higher numbers of foodborne Salmonella infections in humans (Berends et al., 1996; Wong et al., 2002). Reduction of Salmonella in animal products should thus include monitoring and intervention not only at the farm level, but at all levels of production.
For said reasons, it is desirable to develop a vaccine or vaccine strain that is induces a good immune response, is not responsive to recrudescence and/or that is able to prevent or reduce Salmonella recrudescence in order to keep the Salmonella bacteria and the number of Salmonella contaminated animals from increasing during periods of stress. 